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<body class="article">
<div id="header">
<h1>hashmap API</h1>
</div>
<div id="content">
<div id="preamble">
<div class="sectionbody">
<div class="paragraph"><p>The hashmap API is a generic implementation of hash-based key-value mappings.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_data_structures">Data Structures</h2>
<div class="sectionbody">
<div class="dlist"><dl>
<dt class="hdlist1">
<code>struct hashmap</code>
</dt>
<dd>
<p>
        The hash table structure. Members can be used as follows, but should
        not be modified directly:
</p>
<div class="paragraph"><p>The <code>size</code> member keeps track of the total number of entries (0 means the
hashmap is empty).</p></div>
<div class="paragraph"><p><code>tablesize</code> is the allocated size of the hash table. A non-0 value indicates
that the hashmap is initialized. It may also be useful for statistical purposes
(i.e. <code>size / tablesize</code> is the current load factor).</p></div>
<div class="paragraph"><p><code>cmpfn</code> stores the comparison function specified in <code>hashmap_init()</code>. In
advanced scenarios, it may be useful to change this, e.g. to switch between
case-sensitive and case-insensitive lookup.</p></div>
</dd>
<dt class="hdlist1">
<code>struct hashmap_entry</code>
</dt>
<dd>
<p>
        An opaque structure representing an entry in the hash table, which must
        be used as first member of user data structures. Ideally it should be
        followed by an int-sized member to prevent unused memory on 64-bit
        systems due to alignment.
</p>
<div class="paragraph"><p>The <code>hash</code> member is the entry&#8217;s hash code and the <code>next</code> member points to the
next entry in case of collisions (i.e. if multiple entries map to the same
bucket).</p></div>
</dd>
<dt class="hdlist1">
<code>struct hashmap_iter</code>
</dt>
<dd>
<p>
        An iterator structure, to be used with hashmap_iter_* functions.
</p>
</dd>
</dl></div>
</div>
</div>
<div class="sect1">
<h2 id="_types">Types</h2>
<div class="sectionbody">
<div class="dlist"><dl>
<dt class="hdlist1">
<code>int (*hashmap_cmp_fn)(const void *entry, const void *entry_or_key, const void *keydata)</code>
</dt>
<dd>
<p>
        User-supplied function to test two hashmap entries for equality. Shall
        return 0 if the entries are equal.
</p>
<div class="paragraph"><p>This function is always called with non-NULL <code>entry</code> / <code>entry_or_key</code>
parameters that have the same hash code. When looking up an entry, the <code>key</code>
and <code>keydata</code> parameters to hashmap_get and hashmap_remove are always passed
as second and third argument, respectively. Otherwise, <code>keydata</code> is NULL.</p></div>
</dd>
</dl></div>
</div>
</div>
<div class="sect1">
<h2 id="_functions">Functions</h2>
<div class="sectionbody">
<div class="dlist"><dl>
<dt class="hdlist1">
<code>unsigned int strhash(const char *buf)</code>
</dt>
<dt class="hdlist1">
<code>unsigned int strihash(const char *buf)</code>
</dt>
<dt class="hdlist1">
<code>unsigned int memhash(const void *buf, size_t len)</code>
</dt>
<dt class="hdlist1">
<code>unsigned int memihash(const void *buf, size_t len)</code>
</dt>
<dd>
<p>
        Ready-to-use hash functions for strings, using the FNV-1 algorithm (see
        <a href="http://www.isthe.com/chongo/tech/comp/fnv">http://www.isthe.com/chongo/tech/comp/fnv</a>).
</p>
<div class="paragraph"><p><code>strhash</code> and <code>strihash</code> take 0-terminated strings, while <code>memhash</code> and
<code>memihash</code> operate on arbitrary-length memory.</p></div>
<div class="paragraph"><p><code>strihash</code> and <code>memihash</code> are case insensitive versions.</p></div>
</dd>
<dt class="hdlist1">
<code>unsigned int sha1hash(const unsigned char *sha1)</code>
</dt>
<dd>
<p>
        Converts a cryptographic hash (e.g. SHA-1) into an int-sized hash code
        for use in hash tables. Cryptographic hashes are supposed to have
        uniform distribution, so in contrast to <code>memhash()</code>, this just copies
        the first <code>sizeof(int)</code> bytes without shuffling any bits. Note that
        the results will be different on big-endian and little-endian
        platforms, so they should not be stored or transferred over the net.
</p>
</dd>
<dt class="hdlist1">
<code>void hashmap_init(struct hashmap *map, hashmap_cmp_fn equals_function, size_t initial_size)</code>
</dt>
<dd>
<p>
        Initializes a hashmap structure.
</p>
<div class="paragraph"><p><code>map</code> is the hashmap to initialize.</p></div>
<div class="paragraph"><p>The <code>equals_function</code> can be specified to compare two entries for equality.
If NULL, entries are considered equal if their hash codes are equal.</p></div>
<div class="paragraph"><p>If the total number of entries is known in advance, the <code>initial_size</code>
parameter may be used to preallocate a sufficiently large table and thus
prevent expensive resizing. If 0, the table is dynamically resized.</p></div>
</dd>
<dt class="hdlist1">
<code>void hashmap_free(struct hashmap *map, int free_entries)</code>
</dt>
<dd>
<p>
        Frees a hashmap structure and allocated memory.
</p>
<div class="paragraph"><p><code>map</code> is the hashmap to free.</p></div>
<div class="paragraph"><p>If <code>free_entries</code> is true, each hashmap_entry in the map is freed as well
(using stdlib&#8217;s free()).</p></div>
</dd>
<dt class="hdlist1">
<code>void hashmap_entry_init(void *entry, unsigned int hash)</code>
</dt>
<dd>
<p>
        Initializes a hashmap_entry structure.
</p>
<div class="paragraph"><p><code>entry</code> points to the entry to initialize.</p></div>
<div class="paragraph"><p><code>hash</code> is the hash code of the entry.</p></div>
</dd>
<dt class="hdlist1">
<code>void *hashmap_get(const struct hashmap *map, const void *key, const void *keydata)</code>
</dt>
<dd>
<p>
        Returns the hashmap entry for the specified key, or NULL if not found.
</p>
<div class="paragraph"><p><code>map</code> is the hashmap structure.</p></div>
<div class="paragraph"><p><code>key</code> is a hashmap_entry structure (or user data structure that starts with
hashmap_entry) that has at least been initialized with the proper hash code
(via <code>hashmap_entry_init</code>).</p></div>
<div class="paragraph"><p>If an entry with matching hash code is found, <code>key</code> and <code>keydata</code> are passed
to <code>hashmap_cmp_fn</code> to decide whether the entry matches the key.</p></div>
</dd>
<dt class="hdlist1">
<code>void *hashmap_get_from_hash(const struct hashmap *map, unsigned int hash, const void *keydata)</code>
</dt>
<dd>
<p>
        Returns the hashmap entry for the specified hash code and key data,
        or NULL if not found.
</p>
<div class="paragraph"><p><code>map</code> is the hashmap structure.</p></div>
<div class="paragraph"><p><code>hash</code> is the hash code of the entry to look up.</p></div>
<div class="paragraph"><p>If an entry with matching hash code is found, <code>keydata</code> is passed to
<code>hashmap_cmp_fn</code> to decide whether the entry matches the key. The
<code>entry_or_key</code> parameter points to a bogus hashmap_entry structure that
should not be used in the comparison.</p></div>
</dd>
<dt class="hdlist1">
<code>void *hashmap_get_next(const struct hashmap *map, const void *entry)</code>
</dt>
<dd>
<p>
        Returns the next equal hashmap entry, or NULL if not found. This can be
        used to iterate over duplicate entries (see <code>hashmap_add</code>).
</p>
<div class="paragraph"><p><code>map</code> is the hashmap structure.</p></div>
<div class="paragraph"><p><code>entry</code> is the hashmap_entry to start the search from, obtained via a previous
call to <code>hashmap_get</code> or <code>hashmap_get_next</code>.</p></div>
</dd>
<dt class="hdlist1">
<code>void hashmap_add(struct hashmap *map, void *entry)</code>
</dt>
<dd>
<p>
        Adds a hashmap entry. This allows to add duplicate entries (i.e.
        separate values with the same key according to hashmap_cmp_fn).
</p>
<div class="paragraph"><p><code>map</code> is the hashmap structure.</p></div>
<div class="paragraph"><p><code>entry</code> is the entry to add.</p></div>
</dd>
<dt class="hdlist1">
<code>void *hashmap_put(struct hashmap *map, void *entry)</code>
</dt>
<dd>
<p>
        Adds or replaces a hashmap entry. If the hashmap contains duplicate
        entries equal to the specified entry, only one of them will be replaced.
</p>
<div class="paragraph"><p><code>map</code> is the hashmap structure.</p></div>
<div class="paragraph"><p><code>entry</code> is the entry to add or replace.</p></div>
<div class="paragraph"><p>Returns the replaced entry, or NULL if not found (i.e. the entry was added).</p></div>
</dd>
<dt class="hdlist1">
<code>void *hashmap_remove(struct hashmap *map, const void *key, const void *keydata)</code>
</dt>
<dd>
<p>
        Removes a hashmap entry matching the specified key. If the hashmap
        contains duplicate entries equal to the specified key, only one of
        them will be removed.
</p>
<div class="paragraph"><p><code>map</code> is the hashmap structure.</p></div>
<div class="paragraph"><p><code>key</code> is a hashmap_entry structure (or user data structure that starts with
hashmap_entry) that has at least been initialized with the proper hash code
(via <code>hashmap_entry_init</code>).</p></div>
<div class="paragraph"><p>If an entry with matching hash code is found, <code>key</code> and <code>keydata</code> are
passed to <code>hashmap_cmp_fn</code> to decide whether the entry matches the key.</p></div>
<div class="paragraph"><p>Returns the removed entry, or NULL if not found.</p></div>
</dd>
<dt class="hdlist1">
<code>void hashmap_iter_init(struct hashmap *map, struct hashmap_iter *iter)</code>
</dt>
<dt class="hdlist1">
<code>void *hashmap_iter_next(struct hashmap_iter *iter)</code>
</dt>
<dt class="hdlist1">
<code>void *hashmap_iter_first(struct hashmap *map, struct hashmap_iter *iter)</code>
</dt>
<dd>
<p>
        Used to iterate over all entries of a hashmap.
</p>
<div class="paragraph"><p><code>hashmap_iter_init</code> initializes a <code>hashmap_iter</code> structure.</p></div>
<div class="paragraph"><p><code>hashmap_iter_next</code> returns the next hashmap_entry, or NULL if there are no
more entries.</p></div>
<div class="paragraph"><p><code>hashmap_iter_first</code> is a combination of both (i.e. initializes the iterator
and returns the first entry, if any).</p></div>
</dd>
<dt class="hdlist1">
<code>const char *strintern(const char *string)</code>
</dt>
<dt class="hdlist1">
<code>const void *memintern(const void *data, size_t len)</code>
</dt>
<dd>
<p>
        Returns the unique, interned version of the specified string or data,
        similar to the <code>String.intern</code> API in Java and .NET, respectively.
        Interned strings remain valid for the entire lifetime of the process.
</p>
<div class="paragraph"><p>Can be used as <code>[x]strdup()</code> or <code>xmemdupz</code> replacement, except that interned
strings / data must not be modified or freed.</p></div>
<div class="paragraph"><p>Interned strings are best used for short strings with high probability of
duplicates.</p></div>
<div class="paragraph"><p>Uses a hashmap to store the pool of interned strings.</p></div>
</dd>
</dl></div>
</div>
</div>
<div class="sect1">
<h2 id="_usage_example">Usage example</h2>
<div class="sectionbody">
<div class="paragraph"><p>Here&#8217;s a simple usage example that maps long keys to double values.</p></div>
<div class="listingblock">
<div class="content">
<pre><code>struct hashmap map;

struct long2double {
        struct hashmap_entry ent; /* must be the first member! */
        long key;
        double value;
};

static int long2double_cmp(const struct long2double *e1, const struct long2double *e2, const void *unused)
{
        return !(e1-&gt;key == e2-&gt;key);
}

void long2double_init(void)
{
        hashmap_init(&amp;map, (hashmap_cmp_fn) long2double_cmp, 0);
}

void long2double_free(void)
{
        hashmap_free(&amp;map, 1);
}

static struct long2double *find_entry(long key)
{
        struct long2double k;
        hashmap_entry_init(&amp;k, memhash(&amp;key, sizeof(long)));
        k.key = key;
        return hashmap_get(&amp;map, &amp;k, NULL);
}

double get_value(long key)
{
        struct long2double *e = find_entry(key);
        return e ? e-&gt;value : 0;
}

void set_value(long key, double value)
{
        struct long2double *e = find_entry(key);
        if (!e) {
                e = malloc(sizeof(struct long2double));
                hashmap_entry_init(e, memhash(&amp;key, sizeof(long)));
                e-&gt;key = key;
                hashmap_add(&amp;map, e);
        }
        e-&gt;value = value;
}</code></pre>
</div></div>
</div>
</div>
<div class="sect1">
<h2 id="_using_variable_sized_keys">Using variable-sized keys</h2>
<div class="sectionbody">
<div class="paragraph"><p>The <code>hashmap_entry_get</code> and <code>hashmap_entry_remove</code> functions expect an ordinary
<code>hashmap_entry</code> structure as key to find the correct entry. If the key data is
variable-sized (e.g. a FLEX_ARRAY string) or quite large, it is undesirable
to create a full-fledged entry structure on the heap and copy all the key data
into the structure.</p></div>
<div class="paragraph"><p>In this case, the <code>keydata</code> parameter can be used to pass
variable-sized key data directly to the comparison function, and the <code>key</code>
parameter can be a stripped-down, fixed size entry structure allocated on the
stack.</p></div>
<div class="paragraph"><p>See test-hashmap.c for an example using arbitrary-length strings as keys.</p></div>
</div>
</div>
</div>
<div id="footnotes"><hr /></div>
<div id="footer">
<div id="footer-text">
Last updated 2014-07-22 12:39:19 PDT
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